Monoclonal antibodies reactive with epitopes encoded by the viral oncogene, v-fms, of McDonough feline sarcoma virus (SM FeSV) have been prepared. The classes and reactive specificities of these antibodies have been characterized. Biochemical analysis and subcellular localization studies of the proteins encoded by v-fms have been facilitated by use of these reagents. Such studies have indicated the presence of detectable levels of a potential normal cellular analog of the v-fms encoded proteins in various cell lines of human and other species origin.